(1)Field of the Invention
The present invention relates generally to thermal transfer overcoat technology.
(2) Description of Related Art
In thermal transfer overcoat technology, a thin film is produced on a document to provide durability and a glossy finish. A generic thermal transfer overcoat apparatus 100 is illustrated by FIG. 1 (Prior Art). An automatic document feeder (“ADF”) 101, as would be known in the art, feeds a pre-printed document (represented by the so-labeled arrow and horizontal line) into a nip between a pressure roller 103 and a heat roller 105. An overcoat film 107 from a film supply reel 109 is threaded through the same nip. The film 107 is generally a thermally-transferable adhesive laminate material, activated by the heat roller 105, to form a clear overcoat on the printed surface of the document. The laminate generally includes a backing, viz, an expendable carrier ribbon, a clear coating material, and an intermediate adhesive release layer. After passing through the nip, a peel bar device 111 downstream of the nip separates the backing of the film 107 away from the now-overcoated document 113 (represented as two parallel lines). A film take-up reel 115 receives the film backing material. The now-overcoated document 113 is transported to the apparatus output in a known manner.
One goal of the thermal transfer overcoat is to produce an overcoated image on a print medium substrate that does not have any extra overcoat material—known in the art as “tags”—extending over an edge of the document substrate. The tag phenomenon occurs when the cohesive strength of the overcoat material itself is greater than the peeling release force holding the overcoat to its carrier ribbon. The phenomenon is most prevalent at the trailing edge. The tag may be manifested as flakes of coating hanging from the edge of the coated substrate. These flakes are unsightly and can contaminate the coated document, yielding print quality problems. Moreover, the flakes can break off and contaminate the apparatus mechanism, creating reliability problems. Still further, the flakes may pose a hazard to the user as they may be breathed in or adhered to skin or clothing by static electricity forces, and may be even rubbed into the eyes. In general, the attaining of clean edges requires a force that keeps the excess overcoating layer of the film on the carrier at peeling.
FIGS. 2A and 2B (Prior Art) show the perpendicular peel approach where a peel bar 111 is perpendicular to the direction of travel (see, so-labeled arrows) of the document 113 and the film 111. In the prior art, in those areas where there is no substrate, planar peeling-action forces are imposed by the peel bar device 111.
One idea for obtaining a clean edge is described in U.S. Pat. No. 5,555,011 (Tang). A transport system moves a dye-donor web and the receiver medium in a reverse direction along their respective path such that the area of the laminate material which is transferred to the receiver medium breaks cleanly at the trailing edge from a non-laminated area of the laminate material that remains on the dye-donor web as the web support separates from the receiver medium.
A mechanically simpler, easily implemented, low cost, reliable, and effective alternative has been discovered and is described herein.